1. Field of the Invention
The present invention relates to an acceleration sensor configured to detect, in a state where a weight member is connected to a distal end of a vibrating portion of a cantilever beam and the vibrating portion is vibrated at a resonant frequency of the natural vibration thereof, a magnitude of acceleration from a change in a resonant frequency of the natural vibration of the vibrating portion, which is generated by applying acceleration to the weight member.
2. Description of the Related Art
There is known an acceleration sensor for detecting a magnitude of acceleration from a change in a resonant frequency of the natural vibration of a vibrating portion. Related-art acceleration sensors are described below.
An acceleration sensor according to a first related art (see, e.g., Japanese Unexamined Patent Application Publication No. 2005-249446) includes an acceleration sensor device 101 and a control circuit (not illustrated). FIG. 9A is a plan view, looking at an X-Y plane, of the acceleration sensor device 101 that constitutes the acceleration sensor according to the first related art. The acceleration sensor device 101 includes a frame 102, holding members 103A, 103B, 103C and 103D, a support member 104, and vibrating plates 105A, 105B, 105C and 105D. In the following description, an axis extending in the lengthwise direction of the vibrating plates 105A and 105B of the acceleration sensor device 101 is defined as an X-axis of an orthogonal coordinate system, an axis being perpendicular to the X-axis and extending in the lengthwise direction of the vibrating plates 105C and 105D of the acceleration sensor device 101 is defined as a Y-axis of the orthogonal coordinate system, and an axis being perpendicular to both the X-axis and the Y-axis and extending in the direction normal to the vibrating plates 105A, 105B, 105C and 105D (i.e., the direction of thickness thereof) is defined as a Z-axis of the orthogonal coordinate system.
The frame 102 has an external shape in the form of a rectangular frame. The holding members 103A, 103B, 103C and 103D, the support member 104, and the vibrating plates 105A, 105B, 105C and 105D are arranged inside the frame. The vibrating plates 105A and 105B are arranged along one of diagonal lines of the frame 102. The vibrating plates 105C and 105D are arranged along the other diagonal line of the frame 102. Thus, the vibrating plates 105A, 105B, 105C and 105D are arranged in the X-Y plane to extend from a center position of the acceleration sensor device 101 in directions different through 90° between adjacent two of the vibrating plates. The vibrating plates 105A, 105B, 105C and 105D are each in the form of a beam. Each vibrating plate is connected to the frame 102 at one base portion 106A thereof, i.e., its end portion located on the side nearer to corner of the acceleration sensor device 101, and is connected to the support member 104 at the other base portion 106B thereof, i.e., its end portion located on the side nearer to the center position of the acceleration sensor device 101. The vibrating plates 105A, 105B, 105C and 105D each have a resonant frequency of the natural vibration thereof.
The holding members 103A, 103B, 103C and 103D are each arranged between adjacent two of the vibrating plates 105A, 105B, 105C and 105D. Each of the holding members 103A, 103B, 103C and 103D is in the form of a beam and is formed in a zigzag (meander) shape when looked at in a plan view. Respective one end portions of the holding members 103A, 103B, 103C and 103D are connected to the frame 102, and respective other end portions are connected to the support member 104. Thus, the support member 104 is supported by the holding members 103A, 103B, 103C and 103D to be reciprocally movable in the X-axis direction or the Y-axis direction.
The support member 104 functions as a weight member. The support member 104 supports the vibrating plates 105 in cooperation with the frame 102. The support member 104 is disposed to change the resonant frequencies of the natural vibrations of the vibrating plates 105A, 105B, 105C and 105D. FIG. 9B is a perspective view illustrating, in an enlarged scale, the vibrating plate 105A in the acceleration sensor device 101 that constitutes the acceleration sensor according to the first related art. The vibrating plates 105B, 105C, and 105D have the same structure as that of the vibrating plate 105A.
The vibrating plate 105A includes a silicon (Si) layer 112 formed on a silicon dioxide (SiO2) layer 111, a lower electrode layer 113 formed on the Si layer 112, a piezoelectric thin film layer 114 formed on the lower electrode layer 113, and an upper electrode layer made up of a driving electrode 115A and a detection electrode 115B both formed on the piezoelectric thin film layer 114. The driving electrode 115A is disposed over a region spanning from substantially a center of the vibrating plate 105 in the lengthwise direction thereof to its end portion on the same side as the base portion 106B, and is connected to a take-out electrode (not illustrated). The detection electrode 115B is disposed over a region spanning from substantially the center of the vibrating plate 105 in the lengthwise direction thereof to its end portion on the same side as the base portion 106A, and is connected to a take-out electrode (not illustrated). The take-out electrodes are connected to the control circuit.
In the acceleration sensor according to the first related art, when a driving signal from the control circuit is input to the driving electrode 115A, the piezoelectric thin film layer 114 is caused to expand and contract in its portion, which is positioned in a region where the driving electrode 115A and the lower electrode layer 113 are opposed to each other, upon application of an electric field caused by the driving signal. In such a manner, the vibrating plates 105A, 105B, 105C and 105D are each vibrated. At that time, with the vibration of the vibrating plate 105, pressure is exerted on a portion of the piezoelectric thin film layer 114, which portion is positioned in the region where the detection electrode 115B and the lower electrode layer 113 are opposed to each other, thereby generating electric charges. The generated electric charges are output as a detection signal from the detection electrode 115B. The control circuit drives the acceleration sensor device 101 by using the detection signal such that the vibrating plates 105A, 105B, 105C and 105D are each brought into a state where it is stably driven to vibrate at the resonant frequency of the natural vibration.
When acceleration in the X-axis direction or the Y-axis direction is applied to the acceleration sensor according to the first related art in the state where the vibrating plates 105A, 105B, 105C and 105D are driven and vibrated, the support member 104 is displaced in the X-axis direction or the Y-axis direction by an inertial force generated with the application of the acceleration. Forces acting on the vibrating plates 105A, 105B, 105C and 105D from the support member 104 with the deformation of the support member 104 cause the vibrating plates 105A, 105B, 105C and 105D in the driven and vibrated state to extend (or contract) in the X-axis direction or the Y-axis direction, whereby the resonant frequencies of the natural vibrations of the vibrating plates 105A, 105B, 105C and 105D are changed. Accordingly, the frequency of the detection signal is changed with the changes in the resonant frequencies of the natural vibrations of the vibrating plates 105A, 105B, 105C and 105D. Hence a magnitude of the applied acceleration can be detected from the frequency change of the detection signal.
An acceleration sensor according to a second related art (see, e.g., Study on a Resonance Type Micro Acceleration Sensor (1st Report)”, Journal of Japan Society for Precision Engineering Vol. 74, No. 10, 2008 pp. 1051-1055) includes an acceleration sensor device 201 and a control circuit (not illustrated).
FIG. 10 is a perspective view of the acceleration sensor device 201 that constitutes the acceleration sensor according to the second related art. The acceleration sensor device 201 includes a fixation member 202 and a cantilever beam 203. In the following description, an axis extending in the lengthwise direction of the cantilever beam 203 of the acceleration sensor device 201 is defined as an X-axis of an orthogonal coordinate system, an axis being perpendicular to the X-axis and extending in the widthwise direction of the cantilever beam 203 of the acceleration sensor device 201 is defined as a Y-axis of the orthogonal coordinate system, and an axis being perpendicular to both the X-axis and the Y-axis and extending in the direction normal to the cantilever beam 203 (i.e., the direction of thickness thereof) is defined as a Z-axis of the orthogonal coordinate system.
The fixation member 202 has a rectangular external shape when looked at in a plan view. The cantilever beam 203 is fixed to the fixation member 202. The cantilever beam 203 has one end connected to the fixation member 202 and the other end that is a free end. When acceleration in the Z-axis direction is applied to the acceleration sensor according to the second related art, the cantilever beam 203 is deformed by an inertial force generated with the application of the acceleration. In a region of the cantilever beam 203 nearer to the fixation member 202, an opening 204 having a rectangular shape when looked at in a plan view is formed such that the lengthwise direction of the opening 204 is oriented in the X-axis direction and the widthwise direction of the opening 204 is oriented in the Y-axis direction. In the opening 204, a diaphragm 205 in the form of a cantilever beam is disposed such that the lengthwise direction of the diaphragm 205 is oriented in the X-axis direction and the widthwise direction of the diaphragm 205 is oriented in the Y-axis direction, and such that both end portions of the diaphragm 205 in the X-axis direction are connected to the cantilever beam 203. A vibrator 206 is disposed at one side of the diaphragm 205 in the X-axis direction, and a receiver 207 is disposed at the other side. The vibrator 206 vibrates the diaphragm 205 at a resonant frequency. The receiver 207 converts the vibration of the diaphragm 205 to an electrical signal.
The acceleration sensor according to the first related art detects the acceleration in the X-axis direction and the Y-axis direction, but it cannot detect the acceleration in the Z-axis direction with high sensitivity. The acceleration sensor according to the second related art detects the acceleration in the Z-axis direction. However, the acceleration sensor according to the second related art cannot provide high detection sensitivity because, when looked at in a plan view, the diaphragm 205 is arranged to perpendicularly intersect one side of the external shape of the fixation member 202 to which the cantilever beam 203 is fixed.